1. Field of the Invention
The invention relates to a method for making arcuate coil spring, the resulting springs and the necessary devices for carrying out this method.
2. Description of the Background Information
The arcuate coil springs are used in numerous mechanical systems which, in particular, require effects of returning and/or storing and restituting energy, as is the case, for example, in the clutching mechanisms referred to as "shock absorbing double flywheel", or automatic transmissions, vehicle suspensions, etc.
It is known to manufacture such arcuate springs through methods that consist essentially of a deformation beyond the yield point of the coils of the springs that are previously wound straight.
This deformation or bending is generally undertaken on the side which will constitute the external convex portion of the springs, by means of conical or wedge pieces, which are introduced between the coils.
Of course, numerous operations are provided upstream and downstream of this bending operation.
Before bending, it is for example known to perform a straight winding of the coils, a tempering phase (so-called expansion), the grinding of the ends, a prestress shot blasting of the straight springs, a heating and holding of close wound and hot coils.
In addition, after the aforementioned mechanical bending, the springs are hot stabilized, and are then scaled, i.e., controlled during torque in a given angular deformation, then possibly subjected to an anticorrosive treatment.
However, such a method has the disadvantage of causing marks on the surface of the spring wire, due to the very bending operation, as mentioned. Such marks can constitute the beginning of rupture.
Variations of the aforementioned method can be brought in the order of the operations. Thus, the bending performed in the aforementioned manner, for example, can be undertaken right after the winding, tempering and grinding operations, and then followed by the heating, holding of close wound and hot coils, shot blasting and reheating, to finish with the scaling and the possible anticorrosive coating.
However, while grinding after bending makes it possible to remove the traces of said bending, the incipient rupture remains.
One has also envisioned to perform a bending of the straight springs during a heat treatment, as described for example, in the documents WO-A-9106785 or EP 0 584 474. In the latter document, the spring is hot pressed at the winding temperature.
However, regardless of the method mentioned hereinabove, particularly in view of a subsequent shot blasting operation, the springs do not maintain a proper flatness (i.e., a proper flat contact of a generating line of the arcuate spring on a plane) because of the stresses brought by said shot blasting.
That is why the inventor has envisioned an original bending method, a different order of operations with, in addition, specific devices.